1. Field of the Invention
The present invention relates generally to a soft-start apparatus for a power supply and, more particularly to a soft-start apparatus featuring an increasing pulse width of a pulse width modulation (PWM) signal as the power supply starts up.
2. Description of Related Art
Various power supplies have been widely used to provide a regulated output voltage. FIG. 1 illustrates a block diagram of a traditional power supply comprising a switching circuit 30, a switching controller 10, an output inductor L, an output capacitor C1, a voltage divider formed by resistors R1 and R2, a compensation network formed by a resistor R3 and a capacitor C2. An output terminal OUT of the switching controller 10 generates a switching signal VPWM to control a power switch of the switching circuit 30. Therefore, an output voltage VO of the power supply can be regulated.
The switching controller 10 includes a comparator 162, an oscillator 168, a D-type flip-flop 166 and an error amplifier 160. The oscillator 168 generates a saw signal VSAW, which determines a switching frequency of the switching signal VPWM. A positive input of the error amplifier 160 is supplied with a reference voltage VREF. A negative input of the error amplifier 160 is connected to a feedback terminal FB of the switching controller 10 for receiving a feedback voltage VFB. The error amplifier 160 generates a compensation signal VCOM in response to the reference voltage VREF and the feedback voltage VFB. The comparator 162 turns off the switching signal VPWM via the D-type flip-flop 166 whenever the saw signal VSAW exceeds the compensation signal VCOM. A pulse width of the switching signal VPWM is then determined by the operation of the comparator 162.
Referring to FIG. 1, various waveforms at a start-up transient of the traditional power supply is illustrated in FIG. 2. Since the feedback voltage VFB is extremely low at the start-up transient of the traditional power supply, the error amplifier 160 outputs a high compensation voltage VCOM. This causes a wide pulse width of the switching signal VPWM. That is to say, the switching signal VPWM retains a maximum pulse width when the output voltage VO is insufficient at the start-up transient of the power supply. When the time to build a normal output voltage extends, voltage and current stress of the power switch of the switching circuit 30 will inevitably increases.